298 



THE SPECIAL SENSES. 



a way that the meridians of greatest and least curvature are at 

 right angles to each other. Ordinary astigmatism is of the regular 

 variety, and is usually attributed to a defect in the curvature of the 

 cornea. If the astigmatism is such that the vertical meridian has 

 the greatest curvature it is termed "with the rule," since normally 

 this meridian is slightly more curved than the horizontal one. If, 

 on the contrary, the curvature along the horizontal meridian is 

 greater, the astigmatism is " against the rule." The meridians 

 of greatest and least curvature may not lie in the vertical and 

 horizontal planes, but in some of the oblique planes; but so long as 

 they are at right angles the astigmatism is regular. It is evident 

 that such a condition may be corrected by the use of cylindrical 



Fig. 126. Schema to illustrate the paths of the rays of light in a cornea showing 

 regular astigmatism. (McKendrick.) The lower line of figures represents the section of 

 the cone of light, or the images obtained at different distances. The image varies from a. 

 horizontal to a vertical line, but at no place can a point be obtained at which rays along 

 all meridians are focused. 



glasses so chosen as to increase the curvature along the meridian 

 in which the cornea has the least curvature. An eye that suffers from 

 a marked degree of astigmatism can not focus distinctly at the same 

 time lines that are at right angles to each other; hence the use of 

 a series of lines whose rays fall along the different meridians of the 

 cornea, as shown in Fig. 127, will reveal this defect if it exists. If 

 the eye is directed to the center of intersection of the lines some 

 of the lines appear distinct while those at right angles to them 

 are blurred. A normal eye can be thrown into an astigmatic con- 

 dition by approximating the eyelids closely. In this position the 

 tears make a concave cylindrical lens, which alters the curvature 

 along the vertical meridian. What is known as irregular astig- 

 matism is due to the fact that the meridians of greatest and least 



